The present invention relates to data transmission channels and, more particularly, to a method and apparatus for estimating the length of a transmission line having a low-pass filter characteristic.
In a typical Communication system, analog signals are transmitted from a transmitter to a receiver through a transmission channel. A typical receiver includes an amplifier, an analog-to-digital (A/D) converter and an automatic gain controller. The amplifier amplifies the received AC analog signal, which is then converted by the A/D converter into a series of digital values that can be processed. The automatic gain controller monitors the signal power and adjusts the amplifier gain to restore the received signal to desired power level.
Various types of channels have been used, such as xe2x80x9ctwisted pairxe2x80x9d wire line channels. Such channels are commonly used in digital subscriber loops, home, local and wide area networks, and other applications. It is often desired to have the transmitter or receiver optimized for the length of the channel to mitigate channel effects, such as attenuation of the data signals being passed through the channel.
An existing method of estimating the length of a twisted pair wire line channel is to observe and estimate the received signal power. With this method, the receiver contains a power estimation circuit, which is essentially a power meter, that approximates the signal strength at the receiver. Taking advantage of the fact that the received signal power, Pr, is effected by attenuation of the signal in the cable, which increases with cable length, the received signal power estimate is translated to a channel length estimate Equation 1 shows that the channel length estimate, L, is a function of the received signal power, Pr:
L=f(Pr)xe2x80x83xe2x80x83Eq. 1
The existing method takes advantage of the relation between received signal power and cable length and it drives the cable length estimate from the received signal power, assuming that longer cables introduce more attenuation and hence lower signal strength at the receiver.
The inventors of the present application have found that the main disadvantage of the existing method is that it does not account for fluctuations and variations in the received signal power level due to sources other than cable attenuation. Such sources can include variable gain amplifier (VGA) and/or automatic gain control (AGC) power error, Pagc, cable connector loss Pc and cable-termination mismatch, Pm. The signal power at the receiver can be characterized by:
Pr=Pa+Pagc+Pc+Pm+xe2x80x83xe2x80x83Eq. 2
where Pr is the total signal power reduction and Pa is the reduction in received signal power by attenuation in the cable channel due to Inter-Symbol Interference (ISI) or the low-pass filter nature of a twisted pair cable. The non-cable-attenuation sources of power reduction can be significant, which results in an inaccurate estimate of the cable length. This is especially true for shorter cables where Pa can be much smaller than the other non-cable-attenuation, length independent sources of power reduction.
Improved methods and devices for more accurately and efficiently estimating the cable length in a data transmission channel having a low-pass filter characteristic are desired.
One aspect of the present invention relates to a method of estimating the length of a communications channel transmission line that extends between a transmitter and a receiver and has a low-pass filter characteristic and an impulse response. The method includes receiving a data signal by the receiver from the transmission line and estimating the length of the transmission line as a function of the data signal received by the receiver and the impulse response.
Another aspect of the present invention relates to a channel length estimator, which includes a channel input for coupling to a transmission line having a length. A decision feedback equalizer is coupled to the channel input and includes a plurality of steady-state feedback tap coefficients and an equalizer output. A power estimator is coupled either to the steady-state feedback tap coefficients or to the equalizer output and has an estimated power output. A channel length mapping circuit is coupled to the estimated power output and is adapted to map the estimated power output to an estimate of the transmission line length. Yet another aspect of the present invention relates to a data communications receiver for coupling to a transmission line that has a low-pass filter characteristic and an impulse response. The receiver includes a receiver input, which is adapted to receive a data signal from the transmission line, a receiver output and a detected symbol flow path from the, receiver input to the receiver output. The receiver further includes a length estimator coupled to the detected symbol flow path for estimating the length of the transmission line as a function of the data signal received from the transmission line and the impulse response.